with the Prussiate of Potash. 221 



this acid with the bases, and shall conclude this paper with a de- 

 tail of their general properties, reserving the particular history 

 of each till I shall have more fully explained the nature and com- 

 position of the crystallized acid. 



1st, They are all of a deep red colour, crystallizing in four- 

 sided pyramids and rhomboidal prisms. In minute needles they 

 assume a golden-yellow colour. 



2d, In the moist state, the crystals are liable to decomposi- 

 tion by the agency of heat and light, becoming externally of a 

 greenish colour, and in solution depositing a green sediment. 



3d, They are very soluble in water, but insoluble in alcohol, 

 unless considerably diluted. 



4th, Their solutions when hot and concentrated have a pecu- 

 liar smell, approaching to that of weak chlorine, and, with the 

 exception of the salt of lead, they have all a bitterish taste ; that 

 of lead has the sweet taste of its other salts. 



5th, These solutions are decomposed by sulphuretted hydro- 

 gen, becoming green, and depositing sulphur. Some of the hy- 

 dro-sulphurets have a similar effect, but they are not changed by 

 hydrogen gas. 



6th, Treated in powder with sulphuric acid, they give off 

 chlorine gas. From the salts of barytes, strontian and lead, it is 

 also partially driven off by a gentle heat. 



7th, Their solutions are also decomposed by metallic mer- 

 cury, being changed into green, becoming greenish-yellow, and 

 letting fall a blue precipitate ; the solution no longer giving a 

 red but a white with nitrate of silver. They have likewise a 

 strong action upon metallic iron, coating it immediately with 

 Prussian blue. 



8th, They all give similar precipitates with the metallic 

 oxides. 



9th, When dry, they undergo no change by exposure to the 

 air, the salt of cadmium excepted, which deliquesces. 



